An imaging system, such as camera, captures light emission from an object scene and uses the captured light to construct spatial and chromatic representation of the object scene at an image plane. The image may be recorded by a detector or light-sensitive media. Such imaging systems may be characterized by their operating space and their performance within the operating space.
An imaging system's operating space may include, for example, angular field of view, working distance, and spectral bandwidth. An imaging system's performance may include, for example, spatial resolution, relative illumination across the image plane, and system sensitivity to low light conditions.
When an image of an object is formed by an imaging device, such as a camera, the influence of the imaging device on the optical information can be described by various parameters. For example, the image of a point source will be altered according to the imaging device's point spread function (PSF). The PSF characterizes how an imaging device alters the fine details in an object scene when constructing an image scene. An image exhibits aberrations that are brought about by the device and are not otherwise part of the object. More generally, the image field resolution and contrast will be determined by an imaging device's modulation transfer function (MTF). Both the PSF and the MTF may exhibit wavelength dependencies, system aperture geometry dependencies, and aberration dependencies; i.e., MTF may be different for different wavelengths and different for different aperture geometries, and may depend also on the extent to which the final wavefront is diffraction limited or aberration-limited.
The PSF, the MTF, and other such parameters of real imaging systems account for and include diffraction effects and aberration effects. For example, if an aberration is introduced in an imaging system, both the MTF and the PSF may change, decreasing image quality. A system that is aberration limited across the whole field of view may show improved performance when the aperture is reduced. In such a system, however, one wavelength may be predominantly responsible for off-axis performance deterioration.
Some imaging systems exhibit more aberrations off-axis than on-axis and may exploit vignetting to control off-axis aberrations that would otherwise adversely affect image quality. Vignetting involves selectively stopping peripheral rays from reaching the image plane. For example, coma can be reduced by preventing some rays associated with off-axis field positions from reaching the image plane. These rays can be blocked in regions before and/or after the system aperture stop. The rays may be blocked by insertion of a limiting (vignetting) aperture or by under-sizing a lens that is not located at the system aperture stop. However, in systems that image more than one wavelength where different wavelengths have different intensities, such vignetting may reduce too much light at a low intensity wavelength, so that an image for the low intensity wavelength may not be discernible.